1. Field of the Invention
All documents cited in the present patent application are incorporated by reference in their entirety in the present disclosure.
The present invention relates to polymerizable dental materials with a high flexural modulus.
The present invention relates in particular to low viscosity composites with a flexural modulus of at least 18 GPa and a flexural strength of at least 90 MPa.
These composites comprise a first filler with a mean particle size of 1 to 50 μm in combination with at least one additional filler with a mean particle size of 0.5 to 5 μm and a monomer mixture. Aluminum oxide has emerged as a particularly preferred first filler.
2. Description of the Related Art
Polymerizable organic plastics for use as dental material have been known since the 1930s and the use of one of the most well known dental monomers (Bis-GMA) was in particular described in U.S. Pat. No. 3,066,112. Because of the mechanical properties of the polymers obtained, these cannot be used alone. An improvement in the properties can be obtained using fillers. A review of the development of such composites is given, inter alia, in K.-J. M. Soderholm, “Posterior Composite Resin Dental Restorative Materials”, pp. 139-159, 3M Company, 1985; G. Ott, “Composit-Fullungsmaterialien” [Composite Filling Materials], Ivoclar Vivadent Report No. 5, 1990, U. Salz, “Der gefullte Zahn—ein komplexes Verbundsystem” [The Filled Tooth—A Complex Composite System], Ivoclar Vivadent Report No. 7, 1992, and K. Vogel, “Fullstofftechnologie” [Filler Technology], Vivadent Report No. 18, pp. 18-28, 2007.
The individual components can be chosen and combined depending on the use of the composite materials (composed of the main constituents monomer, filler and initiator for the polymerization). In recent years, in addition to the handling properties, the polymerization shrinkage and the mechanical properties, it is above all the aesthetics which have been to the fore in the development of composites. With regard to their viscosity, composites can be roughly subdivided into two groups: on the one hand into packable (high viscosity) composites and, on the other hand, into flowable (low viscosity) composites (flowables). Due to the higher proportion of monomer in the flowables, the mechanical properties achievable, in particular the flexural modulus, can more likely be assessed as poor; however, the viscosity is clearly lower than with the highly filled composites.
However, it has turned out that it can be more economical to prepare even composites for a rather limited field of application than is the case for the “universal composites”.
In the repair of devitalized teeth, posts are frequently fixed in the roots and the dental crown is then built up on these. In order to be able to insert these posts, the root canals can be appropriately prepared, thus drilled and smoothed out. Due to the curvature of the root canals, a great deal of healthy material of the tooth is naturally lost during the drilling or the curvature of the root is allowed to remain and thinner or more flexible posts are inserted. With thin posts in particular, what now matters is that a composite can be made available for the fixing and for the build-up which exhibits the high mechanical strengths necessary and which can be satisfactorily applied in the thin root canals.
A number of documents are admittedly known from the state of the art, e.g. JP 1026506, JP 61241303, JP 63303906, JP 58069805, U.S. Pat. No. 6,709,271 B2, U.S. Pat. No. 6,837,712 B2, U.S. Pat. No. 6,696,507 B2, U.S. Pat. No. 6,620,861 B1, U.S. Pat. No. 6,395,803 B1, U.S. Pat. No. 5,865,623, U.S. Pat. No. 5,847,025, U.S. Pat. No. 5,710,194, U.S. Pat. No. 5,192,815 and U.S. Pat. No. 4,978,640, which describe the use of aluminum oxide as filler in dental composites or composites with good properties; however, nothing was disclosed in the state of the art which suggests the use of aluminum oxide particles in a particular particle size with simultaneous increase or maintenance of the mechanical strength values and comparatively low viscosity of the composites.
Root canal posts are used in endodontically treated nonvital teeth which exhibit extensive coronal damage. This type of therapy is known as postendodontic treatment.
In the past, it was assumed that, in endodontic treatment, the root canal posts strengthen the root canal structure, which has been pretreated. Nowadays, the root canal posts are used to make available a firm and reliable anchoring for the core build-up. A root canal post is necessary with severe destruction of the coronal tooth structure.
Until recently, root canal posts were still prepared from stainless steel or titanium. In view of the fact that more and more composites and ceramic materials are being used for core build-up, aesthetic considerations are exerting ever more influence over the choice of the post material. Dark metal posts are visible through the composites and translucent ceramic materials, resulting in the natural image of the restorations being spoilt. This has led to the introduction of posts which consist of ceramic or fiber-reinforced composites. Fiber-reinforced composites in particular are becoming more and more accepted.
Usually, the postendodontic procedure (direct restoration) consists of the following stages:
Preparation of the remaining coronal structure and removal of the root canal filling (gutta percha) with Gates, Peeso or Largo reamers
Preparation and enlargement of the root canal up to the necessary depth using the corresponding drill
Adjustment by means of testing of the endodontic post and, if necessary, shortening of the post
Application of fixing composite to the post and insertion into the root canal and curing of the composite (light curing or self-curing)
Build-up of the core with a core build-up composite
One considerable disadvantage of this method is the enlargement of the root canal, for the danger exists of root perforation and weakening of the root structure. A method for postendodontic restoration which no longer includes this stage would eliminate the potential risks mentioned and it would furthermore be markedly more agreeable for the dentist and the patient. Ideally, such a new method would also shorten the handling time.
However, the problem in this connection is that, without enlargement of the root canal, preprepared endodontic posts would not be suitable if they do not exhibit a markedly smaller diameter.
A smaller diameter means, though, a poorer stiffness of the posts and accordingly of the entire reconstruction. A poorer stiffness would be detrimental to the stability of the crown, with the possible emergence of risks of separation or crack formation.
Theoretically, this lack of stiffness due to the smaller dimensions could be compensated for by increasing the flexural modulus of the material.
For example, a post with a diameter of 0.6 mm would be suitable in almost any root canal, whereas normal posts exhibit a diameter of between 1.0 and 2.0 mm. The reduction in the diameter from 1.0-2.0 mm down to 0.6 mm would demand a twofold to threefold increase in its flexural modulus, in order to achieve the same stiffness of the entire reconstruction (post plus core build-up). Conventional fiber-reinforced composite posts are based on glass or quartz fibers and exhibit a flexural modulus of approximately 40 GPa. This means that the material which is used for an appropriately thinner post must exhibit a flexural modulus of more than 80-120 GPa. According to the current state of the art in the field of fiber technology, such properties are, however, only achieved by carbon fibers, crystalline polymer fibers and ceramic fibers (SiC, Al2O3). Due to their colouring (black or yellow) and their high cost, these materials are not, however, attractive for this application. An additional possibility would be use of metal posts; however, these are unattractive and can set off allergies.
On the other hand, the stiffness of the reconstruction from post and core build-up is likewise determined through the core build-up material. Theoretically, the reduced stiffness of the post can be compensated for by increasing the flexural modulus of the fixing and core build-up material to 20 GPa or more.
Normally, build-up composites exhibit a flexural modulus of 6 to 15 GPa; however, the fixing materials exhibit a flexural modulus only in the range from 3 to 8 GPa. Admittedly, there already exist some dental composites with flexural modulus values of 18 GPa or more; however, these exhibit an excessively high consistency and cannot be used as fixing material in the root canal.
To summarize, dental composites are known from the state of the art having a flexural modulus of up to 20 GPa. On the other hand, composites with low viscosities (“flowables”) are also known. In this connection, it is also known that the composites with a high flexural modulus always exhibit a high viscosity (depth of penetration of less than 1 mm). In contrast, flowables have a low viscosity (depth of penetration from 5 to 20 mm); however, the flexural modulus here is only in the region of less than 10 GPa.
Low viscosity composites with simultaneously a high flexural modulus are not known from the state of the art.
It would thus be desirable to have available a dental composite which exhibits a flexural modulus of more than 20 GPa and simultaneously a consistency which allows it to be used as endodontic fixing material, thus exhibits a depth of penetration of more than 5 mm, which can also simultaneously be used as core build-up composite.